In recent years, high intensity lamps, such as mercury lamps, xenon lamps and sodium lamps, used as the light sources of lighting devices such as vehicle headlamps and exterior lighting devices have been replaced with semiconductor light emitting apparatuses (for example, light emitting diodes—LEDs) that have long life and low power consumption. Therefore, there is a demand for higher power LED lighting devices including LEDs as light sources.
Most xenon lamps that are currently in widespread use have an output power of about 200 W to about 2000 W. Therefore, power input to LED lighting devices that are replacing the xenon lamps is also increasing. Recent development shows that the power input to one LED lighting device can be greater than 200 W.
As the power of LED lighting devices increases, the amount of heat generated from the LED light source unit increases. Since the light conversion efficiency of the LED light sources is lowered and life thereof is shortened with an increase in operating temperature, an important task is to develop a cooling structure for reducing the temperature of the LED light sources so as to drive them stably. For example, in a cooling structure proposed in Japanese Patent Application Laid-Open No. 2002-299700, an LED-mounted substrate is pressed against and secured to a metal-made heat dissipating-securing plate by a metal-made heat dissipating cover, and the heat dissipating-securing plate having the LED-mounted substrate secured thereto is disposed in a sealed space formed by a light-transmitting cover and a resin case. A plurality of heat dissipating fins are formed on the heat dissipating-securing plate. In this structure, the heat generated from the LED light sources is transferred to the heat dissipating-securing plate through the LED-mounted substrate and through the heat dissipating cover. The heat transferred to the heat dissipating-securing plate is dissipated into the atmosphere through the heat dissipating fins and the resin case, and the LED light sources are thereby cooled.
Another lighting device disclosed in Japanese Patent Application Laid-Open No. 2006-047914 includes an LED serving as a light source, a heat sink that can cool the LED by an air-cooling or liquid-cooling technique, a light emission circuit configured to apply a current for the LED to emit light, and a water droplet sensor configured to detect whether the lighting device is in the water or not. When the water droplet sensor detects that the lighting device is not in the water, the light emission circuit can control the current to be supplied to the LED, thereby preventing overheating of the LED. On the contrary, when the water droplet sensor detects that the lighting device is in the water, the light emission circuit can control the current to be increased, thereby enhancing the brightness of light emitted from the LED.
A Japanese translation of PCT International Application No. 2007-537490 discloses an air-cooling system utilizing a heat sink and a cooling system utilizing both an air-cooling system and a liquid cooling system.